Hello Flavio,
Hi Henning,
I love to shoot slides and i agree somewhat with the idea that the slides are more versatile in the digital world.
However, while i agree that slide films deliver finer grain (i believe this is a nice byproduct of the reversal itself), i am not so sure that the resolution is vastly superior to E6 films. Also, if Kodak MTF curves and Fuji MTF curves are to be believed, in general print films have higher sharpness (higher modulation transfer for finer details) than slide films.
1. You are partly misinterpreting MTF curves.
2. You are making a mistake lots of photographers do: Believing in data sheets as if they were "bible-like truth" or "sent by good" or "the absolute truth"

.
They are not. Because partly the test methods used for them have (very) little relevance for normal, daily shooting situations (e.g. MTF for object contrast of 1000:1). And partly because - often dependent on the manufacturer / supplier - they are also a marketing instrument. For some companies they are more a marketing instrument than for others. After more than 25 years of scientific film tests I can tell you that some manufacturers are sometimes more on the "optimistc side"

with their data (e.g. Kodak), some are partly very conservative (e.g. Fujifilm) so that you get even better results in real world tests than expected from the tech sheet. And there are also cases in which the given data is simply wrong, and intended to hide certain facts and improve sales (fortunately that case is rare, but it happens).
And you are referring to what you heard / read from others (on the internet) concerning resolution/sharpness of film types. But almost all of these "tests" are based on scanning. And therefore
worthless. Because there is no scanner, including the best drum scanners on the market, which can completely record the full resolution of current films!! They all fail in this respect and record significant less of what is really on film. Concerning resolution scanning is the worst you can do with film. It is the weakest method. With optical printing with APO enlarging lenses and with slide projection with the best projection lenses you get
much better resolution values compared to scanning. I've tested all these imaging chains: All types of scanners, optical enlarging with different lenses, slide projection with different lenses. And if you want to get the by far most precise method to see the full resolution of film you have to use a microscope. I am doing that, too.
Example: Resolution is dependent on object/detail contrast. In my resolution and sharpness tests I am using low to medium object contrasts, because they are really relevant in photography: Because most objects / details we photograph have object/detail contrasts of 1:1.3 to 1:32 (1/3 to five stops).
With Provia 100F, an object contrast of 1:4 (two stops), and my standard 50mm lenses from Nikon and Zeiss at f5,6, I get a system resolution (film+lens) of 125 - 140 lp/mm (125 clearly separated lp/mm, and at 140 lp/mm there is still a contrast difference visible). That is what you see under the microscope.
In projection with the best projection lenses you can transfer about 115-120 lp/mm onto the screen. An excellent result, a negligible loss.
But with the best drum scanners the resolution value is dropping below 100 lp/mm.
And with a Nikon Coolscan 5000 you get only about 60-65 lp/mm!
Now let's compare that with Ektar 100: Under identical test conditions Ektar has a resolution of 90 - 105 lp/mm. With optical enlarging (APO lens) you get about 85 lp/mm on paper. With a drum scanner about 80 lp/mm, and with the Coolscan 5000 about 55-60 lp/mm.
So if you compare on basis of the Coolscan scans, there is little difference. But the conclusion that both films have nearly identical resolution is completely wrong! It is just that the scanner is the limiting factor here. The scanner is destroying the full resolution of the films.
I've discussed my results over the years with lots of other experts in the industry, and we have all got very similar results.
You can take a look at this compilation of film characteristics, take a look at the "sharp" column.
https://asa400.wordpress.com/tag/resolution/
Please forget that source. There is a lot of misunderstanding, misinterpretation, "blind believe" in data sheets, and mixing of some right and some wrong statements. They have never done scientific tests by themselves.
Photo Engineer already indicated that slide films cannot benefit from DIR couplers, like negative films do, and this prevents slide films from getting sharpness increases.
Just put a slide and a negative under a microscope and you will immediately see what is really sharper. In most cases it is the slide. E.g. all Velvias can record about 80 lp/mm already at the very low contrast of 1.6:1. There is no negative film on the market which can do that. You need better sharpness capabilities to achieve such high resolutions at such low detail contrasts. Provia 100F also has superior sharpness at low contrasts.
Unfortunately it is quite often the case here that our dear friend Ron (PE) is misinterpreted. Ron retired at Kodak already in 1997. After that we've had real breakthroughs in reversal film technology: 1999: Provia 100F. Short after that Provia 400F. 2003: Astia 100F, Velvia 100F, Sensia III, Elite Chrome 100, Ektachrome E100G and GX. 2005: Velvia 100. 2007: Provia 400X. Ron has never done scientific (lab) tests of all these films. Others have (me included).
Negative films also have much better color purity, less color shifts with density and so on.
Be careful with such general statements. Provia 100F is hard to beat with its very neutral, accurate colors. The Kodak CN films for example all have a bias towards yellow, warm colour rendition (Kodak policy, they want the films to have that look). That often leads to a cyan cast in the blues. Ron has explained that effect in detail here also some time ago.
It more depends on how the manufacturer
wants a film to look, how they design it. And less whether it is a CN or CR film. Look at Velvia 50, Provia 100F, Astia 100F Ektachrome E100G, Rollei CR 200 etc. : Huge differents in colour rendition between these films! Even Velvia 50, Velvia 100 and Velvia 100F have very significant differences in their colour rendition.
And look at Ektar, Pro 400H, the Portras, Gold 200, the Superias etc.: Also huge differents in colour rendition between these films!
Each of these films has its strengths at certain colours, and is a bit weaker at other colours.
The very nice advantage for us film photographers: We always can immediately change the complete colour rendition of our 'sensor'

.
And you may have a bit better 'colour purity' with CN film under lab conditions. But "theory versus practice": We photographers are living in a different world. Our field is not a photo test-lab, but the lab-infrastructure we use, or our own darkroom at home.
Example:
Send identical exposed reversal films to the best labs, and you will get identical results.
Send identical colour negative films to the best labs and order prints and / or scans, and you will get different results from all the different labs. Because the CN process is always a kind of an interpreting process: Filtration (with optical prints), Scanner which is used, scan software, preferences of the scan operator, used paper type. Lots of different influences. 'Color purity' is a quite theoretical idea in such an environment.
And in the end in most cases the decisive question is only: Do you like the colours you get, or not? Fit the colours the message of your photo?
Also, as you perhaps already know, there movie industry uses negative film almost exclusively, finding much better results with the neg->pos process than with reversal, due to color purity and contrast control.
The movie industry is using negative film for decades. They started that at a time in which CN film was behind in technology to reversal film. "Color purity" therefore was not the reason to use negative film. The main reason was that hundreds or thousands of copies were needed for the movie theatres. Negative film printed / copied on negative film = positive film, which can be projected. And the copy process also adds the needed contrast for optimal projection.
In this industrial scale copy process negative is easier to handle and cheaper. That's it.
I would love to see some of your tests and comparisons. You should post some of these here, perhaps they will contribute to the E6 cause.
I've done it several times here in the past.
Best regards,
Henning